Automatic gain control circuits



June 20, 1939. J. D. BRAILSFORD 2,162,878

AUTOMATIC GAIN CONTROL CIRCUITS:

Filed Jan. 26, 1957 SIG/VAL RECTIFIER FOR 4V6 BIAS INVENTOR JOSEPH D. BRAILS FORD BY M ATTORNEY Patented June 20, 1939 2,162,878

UNITED STATES PATENT OFFEE AUTOMATIC GAIN CONTROL CIRCUITS Joseph Douglas Brailsford, London, England, as

Signor to Radio Corporation of America, a corporation of Delaware Application January 26, 1937, Serial No. 122,359 In Great Britain February 3, 1936 6 Claims. (01. 179-471) This invention relates to gain control circuit back effect upon the valve whose gain is to be arrangements and more particularly, though controlled is varied in dependence upon the connot exclusively, to arrangements for obtaining trol exercised upon the amplification of the auxilanti-fading automatic control of the gain of a iary valve. In other words, in accordance with carrier frequency stage or stages in a radio this invention gain control is obtained by con- 5 receiver. trolling in accordance with gain controlling po- It is common practice at the present time in tentials the resultant feedback efiect of a feedradio receivers to obtain anti-fading automatic back impedance associated with the valve whose carrier frequency gain controlby deriving a unigain is to be controlled this control being exer- 10 directional potential dependent in magnitude cised by means of an auxiliary valve which is 10 upon received signal strength and utilizing this subjected to a varying bias potential dependent potential as grid basis for a so-called variableupon received signal strength. mu carrier frequency valve in the receiver. In the case of automaticgain control the In such gain control arrangements reliance for usual case the gain controlling potentials will 5 gain control is placed upon the curvature of the be potentials obtained in any manner known grid voltage-anode current characteristic of the per se, in dependence upon received signal variable-mu valve which is controlled, the slope strength.

of the characteristic being altered by varying The main advantages of this invention are: the static bias on the valve. The distortion is reduced to a minimum; A defect of a known arrangement as just de- (2) A large range-0f gain control is Obtained; 20

scribed is that the whole of the signal input and voltage is applied between control grid and Th a n c tr ca be ta d y ans cathode of the first valve of the receiver. Thereof quite small controlling potentials. fore, with a very strong input signal the curva- The invention is illustrated in the accomture of the valve characteristic is by no means panying drawing which shows diagrammatically 25 negligible with respect to the signal voltage and two embodiments thereof in Figs. 1 and 2. serious modulation distortion and cross-modula- Referringto Fig. 1 which shows one embodition interference often occurs. This distortion ment of the invention a signal a pl fi r Valve is particularly objectionable because, of course, 1, for example a valve of the type commercially it is the strong signals which are least affected known in the United Kingdom under the trade 30 by other forms of distortion, and are, in the designation Marconi MHl, has its anode 2 concase of a broadcast radio receiver, of most pronected to a source (not shown) of anode potential gram value. through an anode resistance 3 of relatively low The main object of the present invention is value (for the type of valve stated, about 120 to avoid this defect and to provide a method of ohms) the cathode 4 of the valve I bein 00 35 automatic gain control which is such that the nec d t earth thr u h a f k imp nc voltage applied between control grid and cathode 5. The negative terminal of the source of anode of the first valve may always be maintained pote t s e as in the 1181-1911 Y- small with respect to the valve characteristic rier fr qu n y pu potentials are pp i d f m curvature, despite the automatic gain control terminals Ebetween the control g id 7 0f Valve 40 action. By employing the present invention the V I and earth by way of a tuned circuit 8 the stages of a radio receiver succeeding the present input terminals 6 being preferably tapped down valve may be designed to give low distortion upon the coil of the tuned circuit 8 as shown. without having to be designed to accommodate The cathode 4 of the valve l is also connected large changes in input amplitudes. I to the cathode 9 of an auxiliary valve Ill whose 45 According to this invention gain control is control grid H is connected to the anode 2 of effected by utilizing uni-directional gain conthe valve I through a condenser l2. The anode trolling potentials to vary the amplification of an E8 of the auxiliary valve H3 is connected to the auxiliary thermionic valve which receives input positive terminal of the source of anode poten- 0 potentials derived from the output circuit of a tial and automatic gain controlling 'uni-direcvalve whose gain is to be controlled, there being tional potentials, derived in any convenient manan impedance common to the cathode circuits ner known per so from apparatus not shown, are both of the auxiliary valve and of the valve whose applied to the control grid ll of the auxiliary gain is to be controlled, and the whole arrangevalve to through a grid resistance It. Amplified ment being such that the net reaction or feedoutput is taken off at terminals l5 from between the grid ll of the auxiliary valve II] and earth. The feed-back impedance 5 may conveniently be constituted by a tuned circuit.

With this circuit there will be negative feedback voltage set up across the feed-back impedance 5 due to the valve and positive feed-back voltage will be set up across the said impedance 5 due to the auxiliary valve it. The arrangement is such that the positive and negative feed-back voltages cancel one another out when the control bias applied to the control grid l l of the auxiliary valve it is of minimum negative value. A suitable operating adjustment is one in which for a weak signal the net feed-back is zero. When a stronger signal is received the grid ll of the auxiliary valve II! will be biased more negatively, and, owing to the curvature of its characteristic curve, its gain is reduced and, therefore, the positive feed-back will be decreased, the balance of feed-back become negative, and the ratio of output to input reduced. Minimum over-all amplification will be obtained when the auxiliary valve I is biased completely to cut-oif.

In a preferred arrangement shown in Fig. 2 the valve l i. e., the valve whose gain is to be controlled, has its cathode connected through the usual capacity shunted automatic bias resistance combination IS in series with a tuned circuit 5 to earth, the end of this tuned circuit remote from earth being connected through a similar capacity shunted bias resistance IT to the cathode 9 of the auxiliary valve Ill. The anode 2 of the valve is connected to one of the output terminals l5 and is also connected to a source (not shown) of anode potential through a tuned circuit l8 which may contain in addition to the usual tuned variable condenser 19 a blocking condenser Zll, the junction of condensers I9, 20, being earthed. A tapping point low down on the coil in the tuned circuit 18 is connected through a condenser 2! to the control grid I I of the auxiliary valve In which grid is connected to a source (not shown) of gain controlling unidirectional potential through a resistance It. The anode 13 of the auxiliary valve i0 is connected to the positive terminal of the source of anode potential, and suitable positive bias is applied to the screen grid of the main valve, if this be of the screen grid type as shown in Fig. 2. A tuned circuit 8 is connected between the control grid 1 of the main valve and earth (the input connections to this tuned circuit being preferably tapped down on the coil thereof as before) and the tuning condensers or other tuning reactances in the three tuned circuits 8, 5 and i8 are all unicontrolled.

With this circuit the anode resistance of the main valve (apart from that of the feed-back impedance) is high, but since the grid of the auxiliary valve is tapped down the voltage swings upon both valves may be kept very small. The maximum voltage magnification of the main valve l with respect to the grid circuit of the auxiliary valve is reduced to unity for the purpose of feeding the said auxiliary valve-which is solely control valve-and at the same time by taking the output voltage from across the whole of the anode load resistance of the main valve (the output terminals are the anode of the main valve and earth) a large overall output voltage is obtain able.

It has been found experimentally, when using triodes of high slope of characteristics for the main and auxiliary valves, that a range of control of approximately 60 decibels is readily obtainable with the main valve used as the first valve of a radio receiver. 7

What I claim is:

1. In a signal transmission system, a tube provided with at least a control grid, a cathode and an output electrode, an impedance connected to the cathode of said tube, a signal source connected between a control grid and the point on said impedance such that the signal voltage developed across the impedance by current flow of said tube is impressed in degenerative phase between said control grid and cathode, a second tube having at least a cathode, control grid and output electrode, means coupling the second tube control grid to the output electrode of said first tube, means connecting the cathode of the second tube to the cathode side of said impedance whereby the space current of said second tube flows through said impedance and develops signal voltage thereacross which is impressed between the control grid and cathode of the first tube in regenerative phase, a signal output circuit connected solely between the output electrode and cathode of the first tube, and means for varying the gain of said second tube in response to received carrier amplitude variations.

2. A method of receiving modulated carrier waves which includes the steps of amplifying collected waves, deriving alternating current voltages from amplified waves, impressing one of the voltages in degenerative phase upon the amplifier input, impressing a second of the voltages upon said amplifier input in regenerative phase, and controlling the relative magnitudes of the two voltages in response to carrier wave amplitude variations.

3. In combination with a signal amplifier having input and output electrodes, an impedance in the space current path of said tube, means for applying the alternating current voltage developed across said impedance in degenerative phase between the input electrodes of the tube, means for applying signals to said input electrodes, a second tube having said impedance in its space current path, means for applying signals derived from the output of said first tube between the input electrodes of the second tube, a signal output circuit coupled solely to the output electrodes of the first tube, and means for controlling the gain of the second tube.

4. In combination with a signal amplifier having input and output electrodes, an impedance in the space current path of said tube, means for applying the alternating current voltage developed across said impedance in degenerative phase between the input electrodes of the tube, means for applying signals to said input electrodes, a second tube having said impedance in its space current path, means for applying signals derived from the output of said first tube between the input electrodes of the second tube, a signal output circuit coupled solely to the output electrodes of the first tube, and means for controlling the gain of the second tube, said impedance including a resonant circuit tuned to the operating signal frequency.

5. In combination with a signal amplifier having input and output electrodes, an impedance in the space current path of said tube, means for applying the alternating current voltage developed across said impedance in degenerative phase between the input electrodes of the tube, means for applying signals to said input electrodes, a second tube having said impedance in its space current path, means for applying signals derived from the output of said first tube between the input electrodes of the second tube, a signal output circuit coupled solely to the output electrodes of the first tube, and means for controlling the gain of the second tube, said last controlling means including a signal rectifier.

6. In combination with a signal amplifier having input and output electrodes, an impedance in the space current path of said tube, means for applying the alternating current voltage developed across said impedance in degenerative phase between the input electrodes of the tube, means for applying signals to said input electrodes, a

nant circuit tuned to the operating signal fre- 10 quency.

JOSEPH DOUGLAS BRAILSFORD. 

